Happy to see the Knockers used in a fantasy setting - and they aren't malevolent! Knockers are said to be either malevolent spirits that actively chip away at a mine's walls and support beams with their hammers to cause cave-ins or well-meaning tricksters who loved to move around miners tools into different areas but also warned miners with their knocking of an impending cave-in. All these Knockers should get headpats.